Stents are generally cylindrically shaped devices which are radially expandable for implantation into a body lumen for holding open a segment of a blood vessel or other anatomical lumen. Stents have found a particular use in maintaining vessel patency following angioplasty, e.g., in preventing restenosis of the vessel.
Stents are typically inserted into the damaged vessel by mounting the stent on a balloon catheter and advancing the catheter to the desired location in the patient's body, inflating the balloon to expand the stent and then deflating the balloon and removing the catheter. The stent in its expanded condition in the vessel exerts a radial pressure on the vessel wall at the lesion site, to counter any tendency of the vessel to close.
Although a variety of stents have been proposed, none to date has proven to be entirely satisfactory. For example, one problem with prior art stents has been contraction of the stent along its longitudinal length upon radial expansion of the stent. This can cause problems in correctly placing the stent within the vessel.
Another problem with prior art stents has been the limited range of expandability. Some stents expand only to a limited degree, necessitating fabrication of stents in a range of diameters, increasing cost of manufacture and posing difficulty in selecting the proper stent size for the vessel to be treated.
Another problem area has been a lack of control over the final, expanded diameter of the stent. The expansion of the stents is a function of the particular design or configuration and the spring constant and modulus of elasticity of the material used to manufacture the stent. Many stents because of their design and configuration exhibit recoil after expansion, making secure placement of the stent at the treatment site difficult. Poor contact between the stent and the vessel wall not only allows for some closure of the vessel, but can lead to more serious complications including migration of the stent away from the desired location. This problem is not readily solved by attempting to compensate for recoil by selecting an oversized stent, since improper selection may result in a stent which exerts to much force of the vessel, leading to an increase in the possibility of vessel injury, such as dissection or intimal hyperplasia.
Another problem area has been in meeting the requirement that the stent be capable of maintaining the radial rigidity and strength needed to hold open a vessel while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery. Placement of stents often involves advancing the stent-catheter assembly through tortuous vascular paths to the treatment site.
It is also important that the stent have a low-profile for intra-luminal delivery and that it be suited for deployment by a delivery system that is reliable and easy to operate.